While I like the idea of a mod competition, I'm not sure that module reskins could be considered "small". For that matter, your descriptions sound a tad more ambitious than simple reskins. And in only a mere three weeks. Perhaps tone it back to something a bit more manageable.

Perhaps have two categories, reskins and models. For reskins, characters probably make the most sense. Could be a head, clothes/armour, whatever. For models, weapons are pretty straightforward. Perhaps that could be split into two categories, completely new custom models, and altered vanilla models (for people that want to tackle heads, etc.).